This invention relates to synthetic cristobalite and more particularly relates to a low temperature hydrothermal method to form cristobalite in high purity.
Crystalline silica is found in nature in three distinct structures, i.e., quartz, tridymite and cristobalite, as identified by x-ray diffraction analysis with each crystalline structure having polymorphic forms stable in different temperature ranges. In each such crystalline form of silica a silicon atom is bonded to four oxygen atoms in tetrahedral coordination. Each form represents a distinct arrangement of silicon-oxygen tetrahedra into a three-dimensional crystalline network. Generally, crystallization temperature determines which structure is formed. Quartz is formed by crystallizing silica at below about 867.degree. C.; tridymite is formed at crystallization temperature of about 867.degree. C. to about 1470.degree. C.; and cristobalite is formed at temperatures above about 1470.degree. C. Cristobalite exists in two polymorphic forms, i.e., low-cristobalite, also referred to as alpha-cristobalite, and high-cristobalite. High-cristobalite, the stable form of crystalline silica at temperatures above about 1470.degree. C., converts to low-cristobalite when cooled through the temperature range of about 200.degree.-275.degree. C. Low-cristobalite is identified by x-ray diffraction analysis with reference to American Society for Testing and Materials X-Ray Powder Diffraction file number 11-695, incorporated by reference herein, which is based upon a sample prepared by the National Bureau of Standards at 1700.degree. C. from silica gel. American Society for Testing and Materials X-Ray Powder Diffraction file number 4-359 for high-cristobalite is also incorporated by reference herein.
Although low-cristobalite is the stable crystalline form of silica at temperatures below the inversion temperature range of about 200.degree.-275.degree. C., low-cristobalite typically is made by a high temperature process. In such a process high-cristobalite is made first at temperatures usually above 1500.degree. C. and then converted to low-cristobalite during a cooling operation.
U.S. Pat. No. 4,122,025 describes a high temperature roasting process to produce low-cristobalite in which quartz sand is roasted at temperatures in excess of 1200.degree. C. and then cooled to cause high-cristobalite, thus formed, to convert to low-cristobalite. Although this process produces low-cristobalite, it uses expensive and energy intensive thermal cycles. In the process described in U.S. Pat. No. 4,122,025, energy costs may be decreased somewhat by using roasting temperatures below 1700.degree. C.; however, 1500.degree. C. is a practical lower limit due to contamination of the low-cristobalite product with other silica structures such as tridymite and unconverted quartz sand.
One objective of the present invention is to provide a process which does not require process temperatures as high as 1500.degree. C. to produce low-cristobalite substantially free of other crystalline silica structures.
In U.S. Pat. No. 4,107,195, low-cristobalite is identified as a minor impurity in crystalline aluminosilicates produced in one example. In this example aluminosilicates are formed during the crystallization of a silica structure when a certain proportion of the Si.sup.+4 atoms are replaced by atoms of aluminum. Other crystalline silicates are known to be formed when other atoms such as iron, boron, and chromium replace silicon atoms in a silica structure. In contrast to pure silicon, replacement by aluminum, boron, chromium, or iron results in a negative framework charge which requires a cation to be associated with the lattice. If such substitution occurs, the total valence of the metals in these structures can be counterbalanced by addition of other metal atoms, such as alkali and alkaline earth ions. Surprisingly, when boron is present during preparation of synthetic cristobalite by the hydrothermal method of this invention, the crystalline product is without contamination with crystalline silicates containing atoms such as aluminum, boron, iron or chromium incorporated within the silica framework. Another object of this invention is to prepare all-silica materials substantially without incorporation of such other atoms as aluminum, boron, iron or chromium within the lattice structure.
Cristobalite is almost wholly insoluble in water which makes it useful as an insoluble mechanically-cleaning abrasive component in liquid scouring cleaning compositions. Low cristobalite possesses a more desirable combination of high degree of hardness, very high degree of whiteness and lower specific gravity than conventional mineral powders used as abrasive substances. When used as a component of a cleaning composition, it is desired to have cristobalite free of iron and iron compounds. Another aspect of this invention is the development of a hydrothermal method to prepare synthetic cristobalite in a pure form having a very high degree of whiteness and without impurities such as iron.
Crystalline material prepared according to this invention can be used in various processes including; as a component of heat resistant caulking compositions, as catalyst carriers, as the major component of cores for investment casting process, as drying agents, as extracting agents, as a component of high quality ceramics or refractories and as a component of cleaning compositions. It is also desirable in these uses to have high purity cristobalite.